1. Field of the Invention
This invention relates generally to differential protective relays for protection of generators, transformers, or station buses, and more specifically to such differential protective relays incorporating means for overcoming the effects of current transformer saturation and for permitting the trip decision to be made at high speed.
2. Description of the Prior Art
Differential protective relaying systems are well known and frequently used for the protection of generators, transformers, and station buses. These differential systems are based on the principal of balancing or comparing the secondary currents of the current transformers located at the input and output terminals of the protected equipment.
In the basic differential protection scheme current transformers are located on each side of the protected device, and a protective relay is disposed between the current transformers such that no differential current flows through the protective relay under normal conditions because the secondary currents through the current transformers are balanced. When an external fault occurs current flow increases at both the input and output terminals of the protected equipment, but the balance between these currents is maintained. Therefore, the protective relay does not operate for the external fault condition.
When a fault occurs in the protected equipment, the current flow on one side of the protected equipment is reversed, thus upsetting the normal current balance at the protective relay. The unbalanced condition causes a differential current to flow through the protective relay, and the protective relay operates to trip the appropriate circuit breaker.
As can be seen, the performance of the protective relay in a differential scheme depends significantly on the performance of the current transformers associated with the protective relay. The current transformers must interpret, in their secondary windings, the ac current conditions existing in the electrical power network and transmit this information to the protective relay. Any secondary error current in a current transformer on one side of the protected equipment upsets the current balance between the current transformers at each side of the protected equipment and sends a current through the relay's operating coil. If this current exceeds the pick-up setting of the relay, the relay operates to trip the circuit breaker and disconnect the faulty equipment. Thus, small error currents in the current transformers can cause erroneous operation of the protective relay.
The secondary current produced by the current transformer is proportional to the primary current, up to a nominal rating for the current transformer. For primary currents above this rating the current transformer saturates, and the secondary current of the heavily saturated current transformer diverges from this ratio. Saturation is usually due to an external fault, which produces a current transformer primary current that is heavier than normal.
AC saturation of the current transformer is not particularly troublesome since it can be calculated, and compensation provided for the resulting errors in secondary current. If, however, the fault current is asymmetrical, a dc component is present. When this dc component decays slowly because of a long dc time constant (large L/R ratio) transient saturation of the current transformer results. This condition occurs more frequently in the protection of generating station buses where the dc time constant of the circuit is likely to be long. For most substation buses, the time constant is short and no appreciable effect from dc saturation results. The presence of a prolonged dc component produces a severe transient saturation of the current transformer. Although it would be technically possible to design a current transformer that would not saturate, such a current transformer would require a cross-section of iron as much as 100 times larger than current transformers of the standard well-known construction.
To overcome the effects of current transformer saturation and secondary error currents, various well-known features have been added to the simple overcurrent protective relay when used in a differential protection scheme. Restraining (or contact-opening) windings in a differential relay permit more sensitive relay settings. This affords greater protection than is possible with a simple overcurrent relay whose trip settings would otherwise have to be high enough to prevent undesired operation due to current transformer performance under heavy through-fault current (i.e., an external fault). Percentage differential relays have two or more restraining windings. On an external fault, the restraining torque is strong and tends to prevent false tripping due to the differential current through the protective relay caused by saturation effects of the current transformers. On internal faults most of the current in the restraining windings is in opposite directions so the total restraint torque is much less than for an external fault. Some relays with restraining windings are designed to trip when a constant percentage of unbalance exists between the restraining currents. Other relays operate over a variable range of differential current and have a variable percentage characteristic. That is, as the magnitude of the restraining current increases, a greater amount of differential or operating current is required to trip the relay.
Another scheme for overcoming the effects of secondary currents errors is by loading the current transformers with a high-impedance protective relay. (See Applied Protective Relaying, Westinghouse Electric Coporation Relay-Instrument Division, 1979, p. 9-8-9-10.) All the current transformers are connected in parallel with a high impedance protective relay. Under normal conditions the voltage at the relay terminals is approximately zero. For an external fault, the voltage at the relay terminals remains at approximately zero if the current transformers on the source-side and fault-side of the protected equipment are not saturated. However, during severe external faults, one of the current transformer nearest the fault may saturate and no voltage or current is developed in its secondary winding. The other current transformers would then have to force their current into the faulted current transformer and the relay. Since the relay impedance is much higher than that of the saturated current transformer, most of this external fault current flows into the saturated current transformer, preventing the protective relay from operating despite saturation of the current transformer nearest the fault. For an internal fault, the impedances of all the current transformers and the relay are high, presenting a high impedance burden to the current transformers. A high voltage appears at the relay terminals; since this voltage is well above the pick-up setting of the relay the protective relay operates. This scheme of differential protection using a high impedance relay is particularly suited for protecting station buses where the dc component of the short circuit current has a long time constant and causes saturation of the current transformers.
Another scheme for preventing relay tripping caused by current transformer errors uses linear coupler transformers instead of current transformers. (See Applied Protective Relaying, Westinghouse Electric Corporation Relay-Instrument Division, 1979, p. 9-1-9-7.) The linear couplers are air core mutual reactors. They are similar to current transformers in general appearance and structural detail except they have an air core with a permeability of 1.0. Thus, the linear couplers do not saturate or create error currents even when heavy primary current flows. The linear coupler transformer produces a secondary voltage proportional to the applied primary current.
The linear coupler method of differential protection is essentially a voltage differential scheme and, consequently, the linear couplers are connected in series. For an external fault, the sum of the voltages induced in the linear couplers is zero. This occurs because the sum of the currents flowing to the bus is equal to the sum of the currents flowing out of the system. As a result, the relay does not trip. In the case of an internal fault there is a difference voltage that appears at the terminals of a high speed, low energy, linear coupler relay. This difference voltage causes the linear coupler relay to trip instantaneously. Like the high impedance relay scheme, the linear coupler scheme is particularly suited to the protection of station buses where the dc component of short circuit current has a long time constant and causes saturation in conventionally-designed current transformers.
Another prior art bus differential protective relay is described in a pamphlet published by ASEA ("Basic Theory of Bus Differential Protection Type RADSS", Pamphlet RK63-200E, Edition 1, 1978). This protective relay uses conventional current transformers with series-connected diodes to develop currents representative of the positive and negative half-cycle currents in the transmission lines connected to the bus. These currents develop restraint and operate voltages across resistors. The restraint voltage predominates when the total current leaving the bus equals the total current entering the bus. Also, for this condition the differential current is zero. Relay operation is the same for external faults. For external faults that cause current-transformer saturation, the differential current is blocked although it would otherwise be non-zero. For an internal fault the operate voltage predominates and the relay trips.
Yet another prior art scheme is illustrated in FIG. 1. In FIG. 1, reference numeral 200 denotes a station bus that is to be protected by a differential protective relay 218, reference numerals 205 and 224 denote transmission lines connected to the station bus 200, reference numerals 204 and 226 denote current transformers installed on the transmission lines 205 and 224 for deriving secondary currents as the normal direction of current flowing into the station bus 200. Transformers 206 and 222 are connected across the current transformers 204 and 226, respectively. Diodes 208 and 219 have anode terminals connected to first terminals of the transformers 206 and 222, respectively, for synthesizing positive half-cycles from the current waveforms produced by the current transformers 204 and 226. Diodes 210 and 220 have cathode terminals connected to the first terminals of the transformers 206 and 222, respectively, for synthesizing negative half-cycles. The cathode terminals of diodes 208 and 219 are connected together; the anode terminals of the diodes 210 and 220 are connected together. The cathode terminals of diodes 208 and 219 are connected to the anode terminals thereof via a series combination of resistors 212 and 214. Second terminals of the transformers 206 and 222 are connected to a first terminal of a resistor 216; a second terminal thereof is connected to a common terminal between the resistors 212 and 214. The resistor 212 converts the positive half-cycle current into a voltage V.sub.1. The resistor 214 converts the negative half-cycle current into a voltage V.sub.2. The resistor 216 synthesizes the positive and negative half-cycles to obtain a differential voltage V.sub.0.
The differential protective relay 218 discriminates between the occurrence of external and internal faults using the voltage .vertline.V.sub.1 .vertline.+.vertline.V.sub.2 .vertline. (i.e., the absolute value of the positive half-cycles of the currents flowing into and out of station bus 200 plus the absolute value of the negative half-cycles of such currents as a restraining quantity, and the voltage V.sub.0 (i.e., the voltage corresponding to the difference between the absolute value of currents flowing into and out of the station bus 200 as an operating quantity.
Operation of this prior art device is described below. Under ordinary conditions when the current transformers 204 and 226 are not saturated or when there is an external fault, the absolute value of current flowing into the station bus 200 equals the absolute value of the current flowing out of the station bus 200 according to Kirchhoff's law. Namely, V.sub.0 =0, .vertline.V.sub.1 .vertline.+.vertline.V.sub.2 .vertline..noteq.0, and .vertline.V.sub.1 .vertline.=.vertline.V.sub.2 .vertline.. There is no operating quantity and the differential protective relay 218 does not operate.
When there is an internal fault accompanying an external fault, the absolute value of current flowing into the station bus 200 and the absolute value of current flowing out of the station bus 200 are unbalanced, so that V.sub.0 .noteq.0, .vertline.V.sub.1 .vertline.+.vertline.V.sub.2 .vertline..noteq.0 (V.sub.1 .noteq.V.sub.2). If the ratio of the operating quantity to the restraining quantity exceeds a predetermined ratio, the differential protective relay 218 operates. Typically, the prior art relay is designed to operate when the ratio of current flowing out of the station bus 200 to the current flowing into the station bus 200 is 1/2 or less. Using this 1/2 ratio figure, yields a ratio for V.sub.0 to .vertline.V.sub.1 .vertline.+.vertline.V.sub.2 .vertline. of 1 to 3, so that the relay operates when the ratio of V.sub.0 to .vertline.V.sub.1 .vertline.+.vertline.V.sub.2 .vertline. is 1/3 or more.
As discussed above, current transformer saturation causes the current transformer to produce a secondary current during only limited periods of the ac cycle. When the current transformer is saturated, the differential quantity (V.sub.0) is zero. However, during other portions of the ac cycle the ratio of V.sub.0 (the operating quantity) to .vertline.V.sub.1 .vertline.+.vertline.V.sub.2 .vertline. (the restraining quantity) is 1 to 1. When such a ratio exists the relay operates erroneously.
For an external fault in which a dc component is superposed on the fault current, therefore, the determination whether the fault is internal or external must be made within a period of several milliseconds between the occurrence of the fault and current transformer saturation. After the fault is determined to have been external, the prior art protective relay is locked to prevent making a fault determination during saturation.
Compared to the prior art protective relays, the present invention is a solid state differential protective relay, for use with conventional current transformers, that is free from the influence of current transformer saturation and therefore able to determine the fault location at high speed. The present invention can also detect high-impedance bus faults, and is insensitive to transient responses.